Conventional surface mounted semiconductor chip packages are manufactured for assembly on printed wiring boards which are installed into electronic controllers such as computers. Such packages comprise a semiconductor chip mounted on a die pad, wired to lead fingers and encapsulated in plastic. These packages are then soldered or otherwise connected into the printed wiring board in predetermined configurations.
The presence of cracks in an integrated circuit IC package substantially increases the probability that moisture or other contaminants will penetrate the package, reach the surface of the semiconductor die and eventually cause the device to fail. The major cause of IC package cracking is moisture related stress that causes a thermal mismatch between the silicon Si of the semiconductor device, the die pad, or lead frame adjacent the silicon device and the encapsulating plastic mold compound.
Thermal stresses tend to induce delamination of, or cracks in, the plastic encapsulation due to differences in the thermal expansion rates of the plastic encapsulation material and the material from which the die pad is made. A die pad typically expands in all directions slower than the plastic encapsulation and thus tends to pull the plastic encapsulation apart at the weakest points, i.e., the small peripheral edge around the die pad.
Thermal mismatch stress is at its lowest point during package molding where temperatures in the range of 185.degree. C. are encountered. After molding, the package is cooled to room temperature. The plastic will contract more than the silicon chip and lead frame which are stacked together before molding. This difference in contraction puts the interface between the pad and plastic under high stress, resulting in separation and delamination. When there is separation between plastic and mount pad, moisture from the atmosphere eventually fills the space created by the separation. During vapor phase reflow the packages are placed into position on the board which is then subjected to heat from a chemical vapor which reflows all the solder thereon at 215.degree. and above. This high temperature causes the moisture to turn into vapor which exerts a huge pressure inside the package, which can cause the package to blow up.
The packages are also subjected to internal and external thermal stresses throughout their normal operating life which may increase the percentage of delamination of, or cracks in, the plastic encapsulation. Internal thermal stresses are caused by the heat generated internally by the packaged semiconductor device when it is operating. This problem is destined to become more acute as advances in large scale integration produce integrated circuits with ever diminishing geometries, increasing densities and a resulting increase in the amount of heat generated by the semiconductor die per square unit of area. External thermal stresses are caused by the heat generated by operation of the electronic system in which the packages are installed.
One technique to prevent cracks involves incorporating mechanical locks into molded plastic packaged devices to help secure a die pad to the plastic mold compound. Such mechanical locks typically include a continuous rib formed around the periphery of the die pad that extends into the surrounding plastic mold compound. Unfortunately, it has been discovered that as the industry moves toward very thin packaged devices, conventional techniques for mechanically interlocking a die pad and plastic mold compound are insufficient to prevent cracking or chipping of the plastic around the periphery of the die pad.
Other techniques involve roughening up the backside of the die pad or use of a coating on the die pad to increase the adhesion between the mold compound and the mount pad or heat spreader.
Yet another technique utilizes pockets or concave dimples on the bottom of the die pad opposite the semiconductor chip in an attempt to increase the contact area between the plastic encapsulation material and the die pad material. It is more expensive to etch dimples than to use a cost efficient die pad manufacturing technique such as stamping, and while somewhat effective, dimples do not resolve the cracking problem. The dimples provide a greater area for contact between the bottom of the die pad and the plastic encapsulation which only helps keep the bottom of the die pad in position relative to the encapsulation in contact therewith. Unfortunately, the position of the bottom of the die pad relative to the encapsulation in contact therewith has a minimal effect on cracks or delamination caused by differences in thermal expansion rates which tend to force the encapsulation apart, and therefore, cracks still occur.
Still yet another technique involves forming holes in a die pad directly under the semiconductor die. Since the holes are only under the semiconductor die, the net result is the same as the pockets or dimples as previously discussed.